The present invention relates to methods and systems that define feature boundaries in a radiation sensitive medium on a workpiece using a diffraction-type micromirror array, extending to production of patterns and structures on a semiconductor substrate. Workpieces include lithographic masks, integrated circuits and other electronic and optical devices. Particular aspects of the present invention are described in the claims, specification and drawings.
Two main types of radiant energy used to generate patterns for integrated circuit or device production are photon beam(s) and electron beam(s). Systems using multiple scanned photon beams are more generally available than systems using multiple electron beams. Photon or laser pattern generator systems usually are faster but less precise than e-beam systems. Multiple, relatively wide beams in a laser scanning system have different characteristics, than a single electron beam in a vector-driven e-beam system. Embellishments can be used in mask writing with a laser scanning system to compensate partially for the larger beam width of the photon beam.
For direct writing applications, photon-exposing radiation may be preferred, because an electron beam may adversely affect layer properties of the integrated circuit. Both at the substrate and in electron charge-trapping layers of the integrated circuit, electrons that pass through a resist layer that is being patterned may damage or change characteristics of the layer below the resist. These modified characteristics may have undesirable effects on device performance. Photon-based writing devices have the further advantage of generally being faster than electron beam devices.
These inventors are continuing development of a new kind of pattern generator that uses photon-exposing radiation. Instead of using one or more scanned laser beams, the new kind of pattern generator uses a micromirror array, in one embodiment, a spatial light modulator (“SLM”), and a pulsed illumination source to print so-called stamps across the face of a workpiece. The Graphics Engine application referenced above is one of several applications with overlapping inventors that disclose aspects of this new kind of pattern generator. These co-pending applications also teach that other kinds of micromirror arrays that may be used with pulsed illumination to print stamps.
The micromirror array under development relies on diffraction, rather than deflection, to produce contrast in the radiation sensitive medium. By use of diffraction, small movements of micromirrors induce scattering of radiation. The scattering corresponds to so-called destructive interference among components of radiation relayed from a single micro-mirror in an object plane. Apertures and other optical components translate the scattering into gray-scaled intensity variations in an image plane corresponding to the radiation sensitive medium on the workpiece.
A single micromirror in the object plane produces a Gaussian distribution of intensity in the image plane. By various approximations, the intensity distribution of a single micromirror affects an area generally corresponding to a 3×3 or 5×5 grid of micromirrors. Conversely, the intensity of exposing radiation at a spot in the image plane may depend on the orientation of 9 or 25 micromirrors in the object plane.
A radiation sensitive medium, such as resist, at the image plane has some thickness and some opacity. The top and bottom of the medium respond somewhat differently to the exposing radiation. This depends on the characteristics of the medium and the contrast at boundaries between areas intended to be exposed and unexposed. Poor contrast typically produces a trench with a wide top and a narrow bottom or a non-vertical sidewall, corresponding to an iso-exposure profile through the thickness of the medium. A non-vertical sidewall compromises the placement of a boundary, due to variations in medium thickness, in particular, after erosion in etch processes. The variations in medium thickness are large, compared to the allowed critical dimension variations. The sine of a 10 or 15 degree angle is 0.1736 or 0.2588, respectively, which indicates the degree to which non-vertical sidewalls make boundary placement is sensitive to medium thickness.
An opportunity arises to improve placement and/or contrast at boundaries. A range of micromirror operation may influence boundary placement, as may a transfer function between boundary placement and micromirror tilt. Calibration methods also may improve placement and/or contrast at boundaries.